High aspect ratio structures are often used in micro-electro-mechanical systems (MEMS) sensors and actuator designs. High aspect ratio structures improve sensor sensitivity, signal output, signal to noise ratio, and so on. Thus, it is important to confirm the etching quality of high aspect ratio structures.
Current inline process controls are either destructive in nature and/or are ineffective in confirming the etching quality of a high aspect ratio structure. For example, a scanning electron microscope (SEM) can be used to confirm the etching quality of a high aspect ratio structure. However, using a SEM involves a destructive analysis because the wafer has to be cut in order to provide a cross-sectional view capable of being scanned. Besides the destructive nature, the disadvantages of using SEM-type analysis also include higher costs and longer cycle times.
Additionally, an alpha stepper can be used as a process control method to confirm the etching quality of high aspect ratio structures. However, the resolution of this technique is low. Thus, the alpha stepper is unable to scan to the bottom of structures having deep aspect ratios.
Furthermore, a white light interferometer can be used as a process control method to confirm the etching quality of high aspect ratio structures. However, even this non-destructive method is ineffective. For example, problems such as interference and transparency make the use of a white light interferometer undesirable to confirm the etching quality of a high aspect ratio structure.
Accordingly, what is needed is a non-destructive control method to monitor a manufacturing process and determine automatically when structures on a wafer have reached a desired dimension, such as a specified aspect ratio.